1. Field of the Invention
The invention relates to an internal combustion engine in which the compression ratio and valve characteristics are variable.
2. Description of the Related Art
In recent years, various controls for controlling internal combustion engines have been proposed and employed to further improve the engine efficiency. The engine efficiency improves, for example, when the charging efficiency of air-fuel mixtures improves and when the compression ratio at which fuel is burned increases. Thus, it has been proposed to variably control the compression ratio in a combustion chamber, as well as the valve characteristics of the intake and exhaust valves, on the basis of the operation state of the internal combustion engine.
However, as the compression ratio increases as a result of such compression ratio control, the top dead center of the piston approaches the valves. Therefore, in a certain state of the valve characteristics control, the piston and the valves may collide with each other when the piston is at or close to the top dead center. Thus, solutions for preventing such collision between a piston and valves have been sought (for example, refer to Japanese Patent Application Publications No. JP-A-2001-263099 and No. JP-A-2005-83238).
Japanese Patent Application Publication No. JP-A-2001-263099 describes a method for avoiding collision between a valve and a piston, in which, when the compression ratio is made high during the compression ratio control, the timing at which the intake valve is retarded and/or the lift of the intake valve is reduced in order to create a clearance between the intake valve and the piston to prevent collision therebetween. Meanwhile, Japanese Patent Application Publication No. JP-A-2005-83238 describes a method for avoiding collision between a valve and a piston, in which the compression ratio is reduced towards the minimum compression ratio of the variation range of compression ratio in order to increase the distance between the top dead center of the piston and the valve and thereby prevent collision between the valve and the piston.
According to theses methods, however, it is considered that the following problems may occur when the above-stated controls are being implemented to prevent collision between the piston and the valve. Specifically, regarding the former method, when the valve lift of the intake valve is reduced and/or the valve timing of the intake valve is retarded to prevent collision with the piston, the charge efficiency of air-fuel mixture may decrease and thus the engine efficiency. Regarding the latter method, when the compression ratio is reduced towards the minimum compression ratio to prevent collision between the piston and the valve, the temperature of air-fuel mixture may not sufficiently increase, which reduces the fuel economy.